Amplifier



AMPLIFIER Filed Sept. 5, 1951 3 Sheets-Sheet 1 -IN NTOR RUD HOFER ATTORNEY R. HOFER April 9, 1935.

AMPLIFIER Filed Sept. 5, 1931 3 Sheets-Sheet 2 m Anna Alllllll 'IV'VII II II INVEN. RUDOLF HEB BY W ATTORNEY Apr i1 9, 1935'. R HQFER 1,997,407

AMPLIFIER Filed Sept. 5, 19:51 3 Sheets-Sheet s .4279. 7a fly Z6 mammm mnmum llHUHXl H II v INVENTOR RUDOLF HOF I BY W? MW ATTORNEY Patented Apr. 9, 1935 umrsn STATES PATENT OF FlCE tion of miicatiorlin September 5, 1921, Serial No. 561,321

January Claims. (01. 179-171) The present invention is concerned with a for instance, Lu and u in such a way that the method adapted to compensate distortions by regeneration or feed-back of suitable phase relation and size, said distortions being occasioned 5 in thermionic-tube amplifier circuit schemes by the use of frequency-dependent impedances in the plate circuit, by the working within nonlinear curve portions, and the like.

In the drawings, Figure 1 represents a circult arrangement shown in diagrammatic form used to explain the invention;

Figure 2 shows asimpliiied form of the invention wherein the output and input circuits are coupled;

Figure 3 shows a circuit arrangement incorporating the invention whereina load is coupled through a transformer;

Figures 4a and to illustrate diagrammatically,

20 tortions are suppressed;

Figure 5 shows the invention applied to a n sistance coupled amplifier;

two embodiments of the invention wherein dis- Figures 6a and 6b illustrate in diagrammatic Suppose a load, or consuming, device contained in the amplifier circuit has a useful resistance 11. and reactive resistance (reactance) iwL (seeFig. 1); and suppose the problem is to deliver the 30 same useful power inside a relatively great range f be N of requencies If the internal tu resistance is appreciable shut in phase.

lation:

I constant, then there exists the; known re- For the lowest frequency to .be transmitted there is oL-0, and forthe highest frequency uL 1L According to this invention, a potential'is regenerated which is in phase coincidencewith that acting at the impedance as indicated, for

instance, in Fig. 2. If the inductance Lu and the transformer T were free from damping and 45 stray,-or leakage, and if, further, the primary resistance of the transformer has been chosen high compared with win, then there holds good for the .stationary state the following equation: 50 -+1 "J=nm+ n+jn+I- i where a W1 the relationship between the secondary and the 55 primary turns of the transformer. Choosing,

is complied with, then the distorting effect of the impedance will be neutralized.

What should be kept in mind is that the circuit apparently behaves as if no distorting impedance were present, though the actual plate alternating current increases with the frequency. By regeneration, the current, for the same original grid alternating current potential, remains unaltered inside the desired frequency range; as a result the plate. alternating current potential, in the presence of a higher frequency w is increased comparatively with lower frequencies in the ratio:

In practice such a circuit scheme would result in lower regeneration-inasmuch as the regeneration quantities Lg, T are not free from damping. However, L: as a rule is of such low value, while the resistance of the transformer generally will be so high, that compensation of such loss of regeneration, feasible for instance by connecting a resistancein the plate circuit, maybe dispensed with. The transformer can be obviated by feed-back to the grid circuit of the cascade preceding the last, though in that instance the said cascade or stage itself should. not cause any frequencies may be neutralized; In the same manner such detrimental resonance eflects as may be caused (by stray of the transformer) may be eliminated. In order to suppress distortions occasioned by the variability of the internal tube resistance, or an external resistance, it is here s lsested to couple against each'other amplified oscillations in a suitable way. Preferably, the generating and the amplified oscillations of the output circuit are first compared in some suitable manner, and then only the distorted oscillat ons are differentially coupled to a reinforced e g t, this resulting in a stable state, in such a way that a very insignificant amount of residual distorted oscillations suflices for the control and regulation of the desired anti-distortion waves.

When a tube is" modulated beyond the straight portion of its characteristic, then the internal impedance Rrwill vary, this variation occurring in accordance with the prevailing aggregate current. Side by side .with the non-linear distortions of the fundamental wave there arise distorting higher harmonics.

In Figs 4a; and 4b two fundamental embodiments are shown by way of exam e for the suppression of these distortions in the case of a transformer-coupled amplifier. Referring to Fig. 4a the differential coupler resistance Ra: is connected in the secondary circuit, while in the example illustrated in Fig. 41) it is contained in the primary 7 1 circuit. In choosing the dimensions for the same,

the ratio of transformation of the transformer U must be taken into consideration. Ra: comprises an ohmic and an imaginary component the size of which corresponds to the phase angle of the -originally existing circuit quantities (resistances) If conditions are-chosen appropriately the counter E. M. F., will be transferred almost completely to the capacity between the grid and the cathode of the last, or power tube.

i 5 shows an embodiment incorporating the basic idea of the invention as applied to a resistance-coupled amplifier, more particularly speaking to the input stage thereof. In order that the potential practicallyspeaking ,may be wholly brought to the capacity between grid and filament, the differential coupling resistance or 1mpedance Rx, is brought to actupon the grid circuit through a transformer. Essentially more efiicient is the compensation of non-linear distortions if the undistorted component of the useful oscillation is compensated and only the non-linear portion optionallyamplified, is brought in differential coupling in the grid circuit so that the non-linear distortions are almost wholly suppressed without the linear. gain being diminished.

Some. fundamental forms of construction are shown in Fig. 6a and Fig. -6b in diagrammatic tials will be amplified by tube A. But, when nonlinear distortions occur these will in an amplified measure be caused to. become operative in phase opposition in the grid circuit. The stable state is attained when the still existent distortion potentials control so much compensating potential in the grid circuit of the main tube that the predominantbalance of the. distortion is compensated. Fig. 6a' shows schematically an arrange-' ment of this kind. -The balance, or simulation, R1: is united with the bridge through a transformer. In the presence of corresponding stepping up Ra: may be reduced. In order that, on the one hand, amplification by the compensating scheme may not be diminished, while on the other hand, the distorting'potential maybe applied as much as possible to the resistance between grid and cathode, it is pre-supposecl that Rx, as well as the resistance transferred .by the transformer of the preceding cascade, is small compared with the grid-cathode reactance of the main tube. v

In thescheme shown in Fig. 6b the secondary winding of the grid transformer ofthe power, or

end, tube comprises two like parts, only one-half 'of the secondary winding furnishing the grid and compensating potential. I The anti-distorting currents flow through both halves of the secondaryv winding in opposite directions so that reaction upon the previous cascade is practically precluded.

' In the case of variable external resistance Hence, it is here necessary to differentially couple the terminal potentialof the load.

Arrangements of this kind are schematically shown in Figs.' 7a and 7b. The transformer T is assumed to leave the phase conditions practically unaltered. The resistance of the .primary inductance therefore should be high compared with the other resistances in the plate circuit for the lowest frequency to be transmitted. It is also possible to insert a tube for separation as in Fig. 7b. As regards the operation of this arrangement the following may be stated. a

I The undistorted oscillations are countercoupled, and so are theharmonic distorting frequencies due to the variation of the internal tube impedance. The gain decreases by the countercoupling, and the non-linear distortions of 'the tube decrease likewise. .Eflicient correction of distortion according to the method hereinbefore described may beused not only in amplifier circuil; schemes, but al'soin modulator stages of transmitters; all that is required-in that case is a rectifier. I

Fig. 8 shows diagrammatically such a distor tion-correcting scheme comprising a mixing tube arrangement (grid potential telephony). In this connection, generally speaking, the following points must be noted: By altering the grid biasing potential (or the operating potential in parallel tube modulation) at tonal rhythm, theinternal tube impedance is changed, and it is to this alteration of the tube resistance that the arising of the side-band oscillations is due. As long as the modulation curve is straight, the impedance has a constant value forthe sideband currents and the modulation is free from phase shift in reference with the audiofrequency excitation; Hence, the entirecircuit must be simulated in orderthat, after rectification, an audiofrequency potential being in phase coincidence with the ex'citation may be impressed upon the standard bridge.

Referring to Fig. 8, fRs is a simulation of the side-band reactance of the radiofrequency tube. Instead of 'theconnection of the compensator resistance for the outer circuit which is'here not feasible, the rectifier circuit is coupled thereis high compared with fRvHoIa so that the radioby high frequencies presents a rather marked frequency will practically undergo no further phase shift as a result of the rectifier arrangement. Now, if the load of the audiofrequency power tube E is purely ohmic in nature, then the input potential may be compensated by the undistorted oscillations coming from the rectifier in the bridge sothat here again merely the distortion frequencies are induced in the grid circuit by means of the distortion-corrector tube -A in phase opposition, whereby any desired correction of distortion will be attainable according to the gain of the tube A.

In a similar way introduction of distortioncorrecting potential could be effected in the plate circuit of the power tube E even if the compensator potential had been derived from this circuit. As long as the whole phase shift of the radiofrequency circuits coupled with the transmitter and of those provided in the receiver circuits remain insignificant for the side-band frequencies there results furthermore a chance of so modulating the radiofrequency by the use of anti-distortion detectors resembling the detector customary in receiver apparatus, that the result of demodulation at the receiving end is practically free from distortion. In the case of quadratic rectification, as for example considerable non-linear distortions are caused as is well known, after demodulation, whenever the modulation exceeds the limit of, say 20-30 per cent. By employing a so-called quadratic detector (i. e., a detector operating according to a square law), instead of G, Fig. 8, distortions would be obviated almost entirely even in the presence of higher modulations if a similar receiving detector is used.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What is claimed is:

1. In combination with an amplifier including in its output circuit a load introducing distoropposite diagonals of said bridge circuit, an am-.

, plifier for distortion currents, means for connecting the input of the second amplifier across an arm of said bridge, means for connecting the output of said second amplifier to the input circuit of said first amplifier and additional means for coupling the said load to the input circuit of said first amplifier.

3. In combination with an amplifier provided with input and output circuits, an impedance load associated with the output circuit normally giving rise to distortion therein, a bridge circuit, connections between the amplifier input and opposite points of the bridge, a transformer connecting the said load to a pair of conjugate points of the bridge, a second amplifier, means for connecting the input electrodes of the second amplifier across an arm of the bridge, and means for connecting the output electrodes of thesecond amplifier to the input circuit of the first amplifier.

4. In combination with an amplifier provided with input and output circuits, an impedance load associated with. the output circuit normally giving rise to distortion therein, a bridge circuit, connections between the amplifier input and opposite points of the bridge, a transformer connal distortion therein and said circuit as a whole possessing a changing resistance characteristic with variations in the frequency of the signal energy handled thereby giving rise to further signal distortion, means for compensating for all of said distortion comprising a link circuit between the output and input circuits to facilitate the transfer of energy from said output circuit to said input circuit, said link circuit being provided with means to simulate both the signal distorting characteristics of the impedance and the signal distortion due to the variable resistance characteristics of the system and to impress the transferred energy from said output circuit to said input circuit in such phase and magnitude with respect to the normal flow of energy through the system that the distortion of the signal energy is compensated to any desired extent saidlink circuit including a bridge circuit, connections between the amplifier input and opposite points of the bridge, a transformer connecting at least one of the output impedances to a pair of conjugate points of the bridge, a second electronic tube amplifier, means for connecting the input electrodes of the second tube across an arm of the bridge, and means for connecting the output electrodes of the second tube to the input circuit of the amplifier system.

' RUDOLF HOFER. 

